So, the trick to becoming London mayor is to be known - like Becks, Kylie, Caprice, Gazza - by just the one name.

Mr Johnson has been a controversial and colourful character

Mention the name Ken to most residents in the capital and it conjures up only one individual, and it's not Barbie's mate. Even the "red" prefix is no longer needed.

Similarly, the name Boris is now almost universally associated with the scarecrow-haired, bicycling, Liverpool-upsetter who is Boris Johnson. And his prefix, "bonking", is no longer in wide use amongst the tabloids since his personal life stabilised.

He is, quite simply, a celebrity. And in an age when the public often cannot recall what, if anything, certain individuals have ever done to deserve their celebrity status that may be all that matters.

David Cameron may well believe that having a candidate who is a fully paid-up member of the TV quiz show circuit will also be enough to give Ken a run for his money.

But there is a problem with Boris. And it also lies in the way the public react to his name and his celebrity.

Heavyweight job

Prompting a smile is one thing, and will probably work to Boris Johnson's advantage. If it turns to laughter, however, that is another matter.

In person, the 43-year-old Old Etonian is witty, charming and thoughtful.

But the question hanging over him is always whether he is "appropriate" or, more pointedly, "safe enough" for a seriously heavyweight job - and London mayor is often said to be one of the four or five most important and powerful political jobs in the land.

Mr Livingstone will be difficult to defeat

No one doubts his intellect or entirely falls for the upper class buffoon image he seems to have deliberately cultivated .

He is a former editor of the Spectator magazine and has held jobs as shadow arts minister and shadow higher education minister.

But some react very badly to his fogeyish image and, more dangerously, his gaffes. His personal life, which saw him sacked from his first frontbench job, has also proved difficult for the Tories in the past.

He caused one of his biggest stirs when he accepted responsibility for a Spectator article accusing Liverpudlians of "wallowing in victim status" after the murder of hostage Ken Bigley.

His boss, then Michael Howard, ordered him to travel to the city and apologise, which he did to mixed reviews.

Two victories

But he later had to apologise to the people of Papua New Guinea after linking the island state with "cannibalism and chief-killing".

And the sight of him besieged by photographers at last year's Tory conference after an outburst against telly chef Jamie Oliver livened up an otherwise deadly dull event.

Most recently, he was at it again, writing that Portsmouth was "a place that is arguably too full of drugs, obesity, underachievement and Labour MPs".

If he wins the Tory selection he will be up against a man who has won two thumping victories, so toppling him next time is a huge challenge which will require more than just celebrity.

Mr Livingstone has had his own controversies to contend with, however.

Two years ago he was accused of bringing his office into disrepute after he compared a Jewish reporter to a Nazi concentration camp guard. He later won an appeal against the finding by the adjudication panel for England.

He only came to power in London after fighting off determined attempts by Labour - led by then prime minister Tony Blair - to stop him, including throwing him out of the party after he said he would stand against the official Labour candidate as an independent.

Mr Livingstone went on to win the election and Mr Blair later relented and admitted he had been wrong.

He is a formidable campaigner but Mr Johnson clearly believes he has the right political stuff to give him a real fight for the job.

His entry in the race to be the Conservative candidate - which will be decided by London voters - and then the main contest would certainly make for an interesting and unpredictable time.